The blackbody concept serves as a model for the far field emissive behavior of hot bodies. It will be recalled that according to Wien's law, the wavelength λW of which a blackbody emits the most radiant light flux, which flux is here denoted I, is inversely proportional to temperature T° (expressed in ° K): λW=2898 (in μm ° K)/T°.
In the natural state, the far field emissive behavior of a solid hot body is most often very similar to that of a blackbody. Its emission is incoherent, i.e. it is isotropic (Lambertian) overall, and of broad spectral width.
Emissivity ∈(λ, θ) relates the emission spectrum I(λ, θ) of a hot body at a wavelength λ in the direction θ to the emission of a blackbody I0, and is defined by the ratio:∈(λ,θ)=I(λ,θ)/I0(λ,T)I0(λ,T) being the monochromatic radiant flux of a blackbody of temperature T at the wavelength λ and I(λ, θ) that of the object at the same wavelength in the direction θ.
Control of the emission direction is associated with the spatial coherence of the electromagnetic field. The longer the correlation length, the greater the directivity. Under these conditions, thermal sources may behave as veritable antennae. To date, many directional thermal sources have been designed by structuring materials at subwavelength scales.
In 1999 Carminati et al., then Shchegrov et al. (2000) demonstrated that the field associated with a resonant surface mode possesses a high spatial coherence. However, as this field remains confined to the vicinity of the interface, far field thermal emission remains incoherent.
In 2002, Greffet et al. (Greffet et al., 2002) demonstrated that it was possible to export the spatial coherence of these waves to the far field using a surface grating. The structure of such a source 100′ is shown in FIG. 1: it consists of a bulk sample 50′ of SiC, the surface of which has been etched to form a 1D grating. However, one of the weaknesses of this type of source resides in the drift in the emission direction with wavelength. In addition, a high directivity is observed only for the p polarization, the only polarization to which the resonant surface mode couples, the electric field of this polarization having no component along the surface. Thus, for the s polarization the radiation emitted remains incoherent.
To mitigate this drawback, Kollyukh et al. (2003), Ben-Abdallah (2004) and Celanovic et al. (2005) proposed to use single films or microcavities to exploit the presence of Fabry-Perot type guided modes and cavity modes to control the emission pattern simultaneously for both polarization states. Veritable thermal antennae, these structures nevertheless exhibit a low emissivity level.
Other more complex structures have been proposed to improve the degree of coherence of these sources. This is the case for the structures proposed by Lee et al. (2005). These sources, composed of a periodic one-dimensional multilayer structure (1D stack) coupled to a polar material, allow the direction and frequency of emission to be controlled simultaneously for both the s and p polarization states of light. However, the directional control of the emission of this type of planar source remains low.
To mitigate this problem, hybrid structures that associate a number of the features of the structures described above have been developed (Drevillon et al. 2007). Among these structures, mention may be made of that proposed by Battula & Chen (2006). It is here a question of nanostructured multilayer materials composed of a cavity placed between a surface grating and a 1D photonic crystal, i.e. a 1D surface grating. This structure allowed a source having a high spatial but also temporal coherence in the visible and near IR to be obtained. The long coherence length of this structure is attributed on the one hand to the excitation of evanescent waves on the surface of the grating. On the other hand, the photonic crystal acts as a polarizer and suppresses the emission of nonresonant s-polarized radiation that would not be directional, leaving the emissivity high only for the resonant component of the emission. Lastly, the high degree of temporal coherence (therefore the small width) is due to the presence of the cavity that surmounts the photonic crystal and that amplifies the emission at the frequency of the resonant modes.
Joulain & Loizeau (2007) have also demonstrated that it would be possible to obtain a temporally coherent directional thermal source by coupling a surface grating to a simple guide. However, this work relied on a theoretical approach and was limited to one-dimensional gratings.
It has also been envisioned to use metamaterials, artificial composite structures that exhibit a negative dielectric permittivity and a negative magnetic permeability, to control the direction of thermal emissions (Enoch, 2002). However, the absence of natural magnetic resonance in the infrared and difficulties with fabrication have slowed the development of these materials.
However, in 2005 a team of researchers (Zhang et al., 2005) demonstrated that it would be possible to design 3D metamaterials in the near infrared (2 μm) by combining perforated dielectric structures and metal films. An analogous result was also obtained in the far infrared (40 to 60 μm) using composite structures based on gold wire (Wang et al., 2007). In contrast to the structures proposed by Zhang et al. (2005), the optical behavior of which was based on a complex set of interferences between electromagnetic waves in these structures, it is the presence of stationary waves along the wires that allows a negative refractive index to be produced, including for the “s” polarization.
These various thermal sources are not switchable insofar as only a mechanical solution, such as a shutter for example, allows the emission of the entirety of the spectrum to be suppressed (or practically suppressed). The position of the emissions may also be shifted over the spectrum by changing the temperature of the source until the Wien wavelength λw departs so far from the range of interest that the emission weakens, this approach moreover having a high inertia. However, the emission mechanism is in no way suppressed.
Moreover, one solution commonly implemented to obtain a directional thermal source that is directionally modulable in its emission band consists in associating:                a weakly directional IR radiation source such as a wire, filament or strip; and        a mechanical device allowing the radiation to be concentrated in a privileged direction, such as a parabolic or paraboloidal reflective surface for example, and the orientation of which may be varied. The use of glass optics is generally precluded in the mid-infrared and optics made of alternative materials (ZnSe, Ge, CsI) suitable for these wavelengths are expensive and fragile.        
The switching (turning on/off) and modulation (change of the emission direction) functionalities obtained in this way have drawbacks in terms of inertia, when one operating state is changed to another, and also in terms of bulk: the optics (the reflective surface) must be much larger than the actual source in order to obtain sufficient directionality.
In summary, there exist directional thermal sources that are not switchable and switchable sources that are not modulable without external devices (shutters, reflectors) and that are moreover rather slow to switch.
Therefore, there remains to this day a need for a non-wire source of infrared radiation that simultaneously meets all of the aforementioned requirements, especially in terms of providing a satisfactory far-field emission level, in terms of directional control of the emissivity, in terms of ease of fabrication, in terms of switching in its emission band, and in terms of low bulk. The field of application where this need is present is notably that of infrared spectroscopy, but also that of heating in any context where a form of agility is required (food-processing, health, control of chemical processes, individual heating of a seated or standing individual, etc.).